1. Field of the Invention
The present invention relates to flame-resistant, non-toxic vinyl polymers which contain phosphazene groups. The polymers of the present invention do not emit any toxic or corrosive products when they are oxidatively degraded.
2. Description of the Prior Art
Presently, there are available a number of flame resistant materials and materials combinations. The vast majority of these, however, produce unacceptably large amounts of toxic and corrosive substances when subjected to oxidative thermal decomposition even in the absence of a flame. The toxic product formation is as great a hazard as a fire itself in any confined location and is particularly dangerous in space capsules, aircraft, or submarines, where egress or ventilation cannot be readily accomplished. The applicability of the few polymeric compositions which are flame resistant and do not form toxic degradation products, on the other hand, is limited because of cost, often poor processibility, and the fact that they do not lend themselves to modifications to improve deficiencies in physical or mechanical characteristics. These materials are based on highly condensed aromatic structures which during oxidative thermal decomposition form chars in high yields, and thus, release combustible decomposition products at too low a rate to support a flame.
All other flame resistant compositions derive this property from the presence of elements, which are known to act as flame retardants. These are elements of the third, fifth, and seventh main groups of the periodic table, specifically boron, nitrogen, phosphorus, antimony, and the halogens. Of these, only nitrogen, phosphorus, and the halogens are directly bonded in or to the polymer backbone. Boron and antimony normally are physically admixed with the flame resistant compositions, the former usually as a salt of boric acid, the latter either as its oxide or oxychloride. Yet internal bonding is preferable to admixture since additives are subject to removal by physical and chemical processes such as abrasion and washing.
The halogenated flame resistant materials such as polytetrafluoroethylene, copolymers of perfluoropropene and vinylidene fluoride, or polyvinylchloride contain the flame retardant bonded to the polymer backbone. Some of these materials exhibit very good flame resistance and have other desirable characteristics such as good mechanical properties and good processibility. However, these materials upon oxidative thermal decomposition produce copious quantities of highly toxic and corrosive gases. Moreover, the thermal decomposition process can take place long before flame temperatures are reached. Polyvinylchloride, for example, was found to lose practically all of the chlorine present (56.7%) in the form of toxic and corrosive hydrogen chloride at about 280.degree. C. leaving a residue which was combusted by excess air in a strongly exothermic reaction (Boettner et al; Organic Coatings and Plastics Chemistry, Preprints, 28, No. 1, 311, April 1968). Polytetrafluoroethylene (Teflon), when exposed to elevated temperature in the presence of air, was shown by K. L. Paciorek et al, Final Report, Part I, Contract NASW-1921, August 1970; CR 114357 and K. L. Paciorek et al, Final Report, Part II, Contract NASW-1921, June 1971 to form carbonyl fluoride, which is hydrolyzed to toxic hydrogen fluoride and carbon dioxide, if water is also present. Fluorinated polymers, which also contain hydrogen, e.g., the copolymer of perfluoropropene and vinylidene fluoride (Viton, Fluorel) release hydrogen fluoride directly if not judiciously compounded. One of the reasons for this behavior is that halogens can be present in a polymer only as singly bonded moieties, and consequently cannot be incorporated into the normally more stable polymer backbone. Accordingly, the fire retarding element can be removed from the material by such simple reactions as dehydrohalogenation. This type of action is unlikely to occur in the case of an element which is either multiply bonded in the backbone or which is a part of an aromatic structure, thus capable of charring without volatilization upon exposure to a flame or elevated temperatures.
The remaining two of the above enumerated elements known to act as flame retardants are nitrogen and phosphorus, both of which are multivalent, thus can be incorporated in a polymer backbone, and are capable of multibonding. The flame retarding capability of triazine type compounds, and especially of phosphorus-nitrogen combinations, has been amply documented. U.S. Pat. No. 2,514,268 (1950); Brit. Pat. No. 638,434 (1950); R. C. Nametz, Ind. Engin. Chem., 59, 99 (1967); G. C. Tesoro et al, 155th ACS Meeting, Organic Coatings and Plastics Chemistry, Preprints, 28, No. 1, 243, April 1968; H. R. Allcock, C&EN, April 22, 1968, 68-81, and C. E. Miles et al, 155th ACS Meeting, Organic Coatings and Plastics Chemistry, Preprints, 28, No. 1, 237, April 1968. The exact nature of their action as flame retardants either alone or in combination is not known. Either one of the elements would be expected to interfere in the free radical chain reactions propagating the flames. More importantly, however, both are known to form strong chars and accordingly do not contribute fuel to the flame. Such chars, in addition, insulate thermally the lower layers of the polymers thus inhibiting pyrolysis, and depress or prevent the access of oxygen to the subsurface. Phosphorus, when contained in an aromatic structure or when substituted by aromatic moieties such as phenyl groups, forms mechanically particularly strong chars, which have been shown to exhibit the above properties.
Flame-resistant polymers containing phosphazene repeating units in the polymer backbone are known, see for example U.S. Pat. Nos. 3,702,837, 3,888,799 and 3,896,058. However, these polymers lack the mechanical properties and good processability possessed by vinyl polymers such as polystyrene. As a result, these polymers have a limited field of use.
Accordingly, there exists a need for flame resistant polymers whose degradation products are both non-toxic and non-corrosive which possesses mechanical and processing properties similar to those of known vinyl polymers.